A LED typically generates light within a narrow spectral range, which can be indicated as a point in a color space. With two LEDs of different color, the human observer will observe a resulting mix color having a color point on the line connecting the two color points corresponding to the two LED colors. The exact position on this line, i.e. the exact mix color, depends on the intensity ratio of the respective light outputs of the respective LEDs, while the intensity of the mix color can be seen as a summation of the respective individual intensities. Likewise, with three LEDs of different color, it is possible to create any mix color within the triangle defined in the color space by the three color points corresponding to the three LED colors. In a typical example, a lighting device comprises three LEDs of red, green and blue colors, respectively, but other color combinations and/or additional colors are also possible. Further, it is known to add a fourth LED, typically generating white light, if increased output intensity is desired.
It is noted that, instead of one LED per color, the device may have a plurality (array, string) of preferably identical LEDs per color, which may be connected in series or parallel and be considered to constitute one light source.
It is noted that the above is commonly known to persons skilled in the art, so a further explanation of this general background art will be omitted.
In a lighting device, the individual intensities of the individual LEDs is controlled by a controller on the basis of an input signal that defines the desired output mix color. Given that the color points of the individual LEDs are known, there is, in the case of a three LED system, a one-to-one correspondence between the output mix color and the individual LED intensities, apart from a common multiplication factor that determines the overall intensity. In the case of four or more LEDs, there are more possibilities for setting the individual LED intensities to obtain the desired output mix color. In any case, on the basis of the input signal that defines the desired output mix color, the controller can determine the individual LED intensities, for instance by consulting a memory that contains information, for instance in the form of a look-up table or a formula, defining a relationship between output color and LED intensities.
A problem in this respect is accuracy and stability. On the basis of the information stored in the memory, the controller is only capable of determining setpoints or target values for the individual LED intensities, which are translated to setpoints or target values for the individual LED control signals generated by the controller. But it may be that the response by a LED to a control signal differs from expectations, for instance as a matter of tolerances or because it changes with time, temperature, etc. If the light output intensity (flux) of a LED is not correct, the resulting output mix color may deviate noticeably from the desired color.
In order to assure that each LED produces the correct intensity, it is necessary to provide for some feedback of the actually produced intensity to the controller. Such feedback can be provided by an optical detector, typically a photodiode. Although it is possible to use individual detectors per LED, a problem would be that different detectors may give different responses. Therefore, it is better to use one single detector with a wide sensitivity range, i.e. a detector sensitive to the different wavelengths produced by the different LEDs. Consequently, since it is intended to measure the individual light output of the individual LEDs, it is necessary to briefly switch off all LEDs except the one being measured. Since LEDs and photodiodes have short response times, a measuring event may take place within a very brief time window and the interruption of the non-measured LEDs may be very short. Nevertheless, the brief interruption of the non-measured LEDs constitutes a reduction of the average light output of these LEDs, and hence a deviation of the output color and reduction of the output light intensity, which, brief as it may be, may be noticeable.
In order to avoid these artefacts, the brief interruption of the light output of the non-measured LEDs during a measuring window is compensated by a brief increase of the light output of the non-measured LEDs outside such measuring window.
A device showing all the above features is disclosed in U.S. Pat. No. 6,445,139, and for a more elaborate background explanation reference is made to this document, of which the content is incorporated herein by reference.
Generally, the light intensity of a LED is proportional to the magnitude of the current through the LED. In the device as disclosed in said document, the light intensities of the LEDs are varied by varying the current magnitude. Thus, a LED is driven with a constant current magnitude, which magnitude is controlled to have a certain desired value. Immediately before and after a measuring window, the current is boosted to have a higher magnitude than the constant desired value. Thus, averaged over a time portion including the duration of the boost and the measuring window, the average current is equal to the desired value and hence the average light intensity is equal to the desired value.